The present invention relates to a projection cathode-ray tube having improved high-temperature characteristics.
At present, there are commercially available color projection video apparatuses in which three high-luminance cathode-ray tube emitting blue, green and red colors a juxtaposed, and the pictures on these tubes are enlarged by optical lenses and projected onto a large screen to reproduce color images thereon. This video apparatus has heretofore frequently been used for reproduction of television pictures for educational and amusement purposes, and it is expected in the future to be broadened in scope of applications as a high definition and high resolution television broadcasting or video system. In this color projection video apparatus, in order to make the brightness on the large screen as high as possible, it is required to apply an electron beam energy of tenfold or more, as compared with a conventional direct-view color cathode-ray tube, onto the luminescent screen of the above cathode-ray tube. Accordingly, the temperature on the luminescent screen is elevated to 60.degree. C. or higher by ordinary operation. Generally speaking, the brightness on the luminescent screen is known to be lowered as the temperature is elevated. Further, the efficiency on the luminescent screen is generally lower under a high density electron beam, with the spot size of electron beam on the cathode-ray tube being about 1/5 of that of the direct-view color cathode-ray tube for obtaining clear images on the enlarges picture screen. Therefore, considerations different from those of the direct-view cathode-ray tube should be taken into account for the cathode-ray tube for the projection apparatus with respect to the structure of the cathode-ray tube and the phosphor constituting the luminescent screen.
For example, a cathode-ray tube is known, in which the temperature elevation of the luminescent screen is suppressed to some extent with a structure capable of holding a water layer on the outside of the cathode-ray tube. Forced air cooling by blasting the air against the outside of the luminescent screen of the cathode-ray tube by means of a fan is also known. However, according to these methods, there are involved the drawbacks such that the structure of the cathode-ray tube becomes complicated or that the production cost is increased.
As the red phosphor in projection CRT's, europium-activated yttrium oxide has been used because yttrium oxysulfide, frequently used in the direct-view color cathode-ray tube, is markedly lowered in emission efficiency at high temperatures. For the blue phosphor, silver-activated zinc sulfide having high emission efficiency has been used. For the green phosphor, manganese-activated zinc silicate or terbium-activated gadolinium oxysulfide has been employed, because a zinc sulfide type phosphor frequently used in the direct-view type color cathode-ray tube is markedly lowered in emission efficiency under the high electron beam energy density.
With reference to reproduction of a white picture synthesized by the red, green and blue emitting phosphors on the projected screen, about 70 percent of its luminance is contributed from the green element, Therefore, among the red-, blue- and green-emitting phosphors, improvement in emission efficiency particularly of the green-emitting phosphor will lead to a high luminance of the color projection video apparatus. Whereas, manganese-activated zinc silicate used for the green emitting phosphor of the prior art is as low as 79% in energy conversion efficiency from input electron beam to light output. Furthermore, it often shows luminescent screen deterioration under electron bombardment, which is so-called "electron burning". Moreover, it has the drawback of long decay time of luminescence after cessation of electron beam excitation which makes moving pictures tent to be tailed on the projected screen. On the other hand, terbium-activated gadolinium oxysulfide has the drawback of marked lowering of efficiency with temperature elevation, although it has high luminescence efficiency at room temperature of 10% or more. Therefore, under ordinary operation, the conventional projection type video device cathode-ray tube employing terbium-activated gadolinium oxysulfide has brightness only comparable to that employing manganese-activated zinc silicate. Further, due to efficiency lowering of the green-emitting CRT with temperature elevation, the projected image changes in color with the initial temperature elevation of the cathode-ray tubes during about 10 minutes after commencement of picture projection, whereby cumbersome color readjustment is required.
In addition to high luminescence efficiency at high temperatures, the following conditions are also necessary with respect to the color image reproduction comparable to that of the direct-view type color cathode-ray tube. As the emitted light from a green phosphor is greater in value x and smaller in value y in the CIE chromaticity diagram, namely more intensified in yellowish hue, the sum of the electron beam energy of blue, green and red cathode-ray tubes becomes smaller when constituting a white picture, whereby the emission efficiency of the video device as a whole is enhanced. On the other hand, for increasing the color reproduction area of pictures, the emitted light is desired to be as near to the edge (i.e., great in saturation degree of color) on the chromaticity diagram as possible. In accordance with the above viewpoint, in the direct-view color cathode-ray tube, the green component is selected so as to exhibit a chromaticity of 0.30&lt;x&lt;0.34 and 0.57&lt;y. Meanwhile, in a projection type, the emitted color of the green phosphor comprising maganese-activated zinc silicate is x=0.23 and y=0.69 to be strongly hued with green, whereby the emission efficiency of the video device as a whole will be lowered. Also, in the case of the phosphor with the use of terbium-activated gadolinium oxysulfide, its emitted color is x=0.325 and y=0.543, thus involving the drawback of a lower degree of saturation (purity).
Other than the green phosphors described above, terbium-activated rare earth oxyhalide phosphors are show to show high efficiency under electron beam or under X-ray excitation. These phosphors are disclosed in Philips Research Report, Vol. 22, page 481, published in 1967. The phosphor host compounds in this paper are lanthanum oxybromide, lanthanum oxychlroide, lanthanum oxyfluoride, yttrium oxyfluoride, yttrium oxychloride and yttrium oxybromide.
Among the above phosphors, particularly lanthanum and gadolinium oxyhalides, J. G. Rabitin discloses in Japanese Kokoku 49-34310 that good results be obtained by applying these materials on the luminescent screen of X-ray image converter through utilization of high efficiency under X-ray excitation possessed by these materials. Particularly, the lanthanum oxybromide phosphor is stated to give the highest emission efficiency by X-ray excitation and suitable for X-ray sensitizing screen. Further, there is a disclosure that the above bromide phosphor also shows high luminescence efficiency and good high-temperature characteristics even by electron beam excitation (see The Extended Abstract No. 306 in the Annual Meeting of American Society of Electrochemistry, in Autumn, 1979). There is also a disclosure that it can be utilized for a monochrome projection video apparatus (emitted color becomes white at low terbium concentration) to give good results (see The Extended Abstract No. 153 in the Annual Meeting of American Society of Electrochemistry, in Spring, 1981).
The present inventors tried to apply lanthanum oxybromide chosen from the above phosphors for the green-emitting projection cathode-ray tube, but could not obtain good results. The emitted color lies at x=0.35 and y=0.57 on the CIE chromaticity diagram, and was too yellowish for the green component required in the present invention. Moreover, when applied to a projection CRT, the luminescence intensity steeply decreases above 80.degree. C. Further, this phosphor is chemically unstable and it was found that adherence strength between the phosphor screen and the glass face-plate of the cathode-ray tube is weak during the phosphor screening process and that it was difficult to obtain a uniform luminescent screen.
The present inventors, since the bromide phosphor proved to be inapplicable for the green-emitting projection CRT for the reasons as mentioned above, have studies further on other rare earth oxyhalide phosphors.
Yttrium oxychlroide, which is stated to have high luminance and high efficiency, was found to be chemically unstable, resulting in difficulty in forming a good luminescent screen. Oxyfluoride phosphors were found to cause reaction with a quartz crucible during phosphor firing process, whereby purity is lowered or mass-production phosphor preparation becomes difficult. Only terbium-activated lanthanum oxychloride was found to suit our objects.